The modern pig industry rely on relatively few feedstuffs mostly from cereals (corn, wheat, barley, oats, rye and rice), cereal co-products (different milling fractions, residues from biofuel and alcohol industries, etc.), cereal substitutes (tapioca, maniocca), legumes (peas, beans, lupins), protein concentrates (meal or cakes of soybean, rape, sunflower, cotton) and co-products from the sugar and starch industries to produce compounds feeds. The classical pig diet can also be characterised as relatively concentrated but an increased demand of high energy cereals for direct human use and increased availability of fibre rich ingredients from, for instance, the feed milling or starch extraction/fermentation industries have promoted an increased utilisation of fibre rich co-products in the pig feeds (Serena et al. 2007). Especially pregnant sows may be supplied with fibre rich diets without compromising their reproductive performance. The direct use of forage crop is also developing although at a rather limited scale and primarily in organic farming. Other benefits, such as increased well being of animals, improvement of the gut transit or reduction of stomach ulcers also favour an increased utilisation of fibre rich ingredients in pig feeds. An increased dietary fibre concentration is on the other hand associated with reduced available energy content of the diet if not combined with high energy ingredients such as animal fat or vegetable oil. The consequence is that the amount of feed required per kg meat produced is increased (Fernández et al. 1983; Lekule et al. 1990). The present chapter will summarise our current knowledge concerning dietary and enteric fermentation that may influence the methane (CH4) emission in pigs. Enteric fermentation is the digestive process by which carbohydrates are broken down by microorganisms into simple molecules for absorption into the blood stream of the animal. The data compilation is based on all experiments carried out in the respiration chambers at Research Centre Foulum during the last 20 years. It should, however, be stressed that none of the studies were specifically designed to investigate dietary or enteric pig factors that may influence the CH4 emission. The data used in the chapter is based on Danish pigs with reference to other European studies. In a global context Denmark’s 12 million pigs (FAOSTAT, 2009) contribute with 1.3 % of the worlds pig population. The main number of pigs is in Asia (59.6 %) where the main pig population stay in China (47.8 % of the worlds pig population). The objective of the chapter is therefore: To obtain a general overview of the pigs’ contribution to methane emission. Where is the pigs’ enteric gas produced and how is it measured. The variation in methane emission and factors affecting the emission. Possibility for reducing the enteric methane emission and the consequences.